Thursday, October 28, 2010

Unusually Well Preserved Crater Found in North Sea

The team used three-dimensional seismic reflection data, collected in the course of routine oil exploration, to build a map of the crater at a resolution higher than that of other similar craters.

The three-dimensional data will enable scientists to study the internal structure of the Silverpit crater. Most study of multi-ringed craters has been done by analyzing photographs of craters on other planets and moons.

The data didn't come cheapa seismic survey can cost U.S. $2 million to $3 million. So, while such data is commonly used in fossil fuel exploration, it is generally too costly for academic researchers.

In an accompanying article in Nature, John G. Spray of the Planetary and Space Science Centre at the University of New Brunswick in Canada said: "This sort of claim is important because we know so little about how impact structures are created when meteorites and comets hit planetary bodies that any new example helps."

"Confirmation that the structure was indeed formed by an impact will require further evidence," Spray cautioned.

Underwater Impact

Analysis of sediments in the crater indicated that at the time of impact, the area was under water of depths from 50 to 300 meters (164 to 984 feet).

"Unlike all of the craters that formed on the shore, this one was formed on a sedimentary basin that was subsiding," Stewart said. Sheltered from the effects of wind and rain, the crater was preserved by "a rain of fine-grained sediment that will fossilize anything on the seafloor," he said.

Stewart said that nearly all meteors and comets that hit Earth are traveling between 20 and 50 kilometers (12 and 30 miles) per second, which suggests the object that created the Silverpit crater was moving that fast.

"What's less well constrained is whether it was a comet or a meteor," Stewart said. "The impactor gets completely obliterated when it hits."

Using established equations, and factoring in the size of the Silverpit crater, Stewart estimated that if it was a meteor that struck, it would have been about 120 meters (394 feet) in diameter and weighed about 2 million tons. If a comet, the object likely would have been larger, as a comet's icy structure is less dense than a rocky meteor.

Stewart said that until a more exact date for the Silverpit impact is determined, he could only speculate on a possible connection between the Silverpit impact and the space object that is thought to have killed off the dinosaurs.

The impact that occurred at the end of the age of the dinosaurs has been dated very precisely to 65 million years ago. The Silverpit crater might have been formed some five million years later.

Multi-Ringed Structure

According to Spray, impact craters show a correlation between size and structurefrom simple bowl-shaped craters formed from small impacts to large multi-ringed structures that are left by the largest meteors. The Silverpit crater is among the smallest known multi-ringed craters.

"You've got to go all the way to the moon to find analagous features," Stewart said.

Previously known multi-ringed craters on Earth, such as the ones at Sudbury, Canada, and Vredefort, South Africa, are more than 250 kilometers (155 miles) in diameter. Extraterrestrial craters, such as the Orientale basin on the Moon, may be as much as 2,000 kilometers (1,243 miles) in diameter.

"Development of multiple concentric rings at such a small diameter may not be unusual because, until recently, we have been unable to obtain images with this degree of detail in such a well-preserved example," Spray said.

Geologists are not certain how multi-ringed craters form. "How these rings form is right to the cutting edge of the research aspect," Stewart said. "It's not something that there's an accepted model for."

Spray said one possible explanation is "Bingham fluid behavior," which causes the concentric ripples around a stone tossed in a pond. Faulting of Earth's surface is more likely to be the cause in the case of Silverpit.

Stewart's hypothesis is that seismic energy created by the impact interacted with sediments, forming concentric fractures around the crater. Over a longer period, these fractures developed into fault zonescracks in the ground that allowed large sections of rock to shift.

Spray said that the discovery of Silverpit highlights what a small sample of craters scientists have available to studyonly about 160.

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